With the design of the M14. And that was to put a full auto switch on it. By doing that, they tried to make it something it never should have been, which is why 90% of them had the switch removed and the "selector lock" installed, making it semi only. If they hadn't fooled with the full auto in the first place, it would have been hailed as a great rifle and a worthy replacement for the M1.

Instead many people just look at its problems as a select fire weapon, and its short service career (as a general issue rifle), which was cut short by politics and changes in what the service wanted, not due to any fault of the rifle's design.

Ditto that,

The first testing of the M-14 showed that the weapon on full auto was uncontrollable. The M-1A is what the M-14 should have been. Almost immediately after it was issued, troops were instructed to not use it on full auto and subsequently the selector switch was rendered inoperable. The rifle was simply too light to fire the 7.62 NATO on full auto aprx. 700 rnds per minute. This was the Army trying to have it all, a light weight weapon that fired a heavy round on full auto, the expectations were simply unreasonable. As a semi-auto weapon it performs outstandingly and it puts 20 rounds of 7.62 x 51 at your fingertip. The M-14 fires a 7.62x 51 which is essentially a Winchester .308 and is very effective at longer ranges. At 500 meters and further it is still capable of penetrating obstacles such as car doors and still make the kill. Its short service life was due entirely to the personal agendas of various people and completely unrelated to the weapons performance. This was painfully and often tragically obvious in the early days of its replacement when the M-16 was still having so many problems. In spite of all of that the M-14 has never been completely out of service. From the time of its inception it has always been in service in one capacity or another. Very often down through the years when ever the Army has wanted to “Reach out and touch someone” they have called up the M-14 as the weapon that is fundamentally up to the task. Even today if you look on discussion boards frequented by our service men you will see that the M-14 is well thought of and jealously desired by many of our service men. This is because in a desert environment it can reach a very long distance and still kill you dead when it gets there.

This is really the same old argument of one caliber / cartridge vs. another. Frankly I have trouble understanding why some people insist on clinging to the idea that the much smaller though significantly faster bullet is more lethal than the much heaver and almost as fast bullet.

Here are some interesting links where you can see what the difference is between these rounds. The first link shows the .223 at the bottom of the page and the second link shows the 7.62 x 51. I think the are some more direct comparisons on that site but I just didn’t have time to look for them.